Ideally when the follicle is around 18-20 mm and on the 13th or 14th day(takingnyour menstruall cyclenis of 28 days) ,the follicle should rupture.the ruptured follicle would release the egg which would meet the sperm and result in conception.
Secondly the thickness ( uterine linning) should ideally be 8-10mm for the pregnancy
to attach itself.
You should ask your doctor to advice you hcg
injections to facilitate the rupture of egg if it doesn't rupture by the 18th day.similarly he will advice you injections to improve the uterine thickness.
In case the follicle doesn't rupture or decrease in size in subsequent scans i advice you a pelvic ultrasound
as soon as you finish with your next periods to rule out the formation of cyst
as sometimes ovarian cysts can be misdiagnosed as follicles or overmature follicles can become cysts.